The invention relates to a method of making MMC components by an infiltration process, with the preform which is disposed inside a crucible and, optionally, held by a preform holder, being placed inside a pressure container, wherein the atmosphere inside the pressure container is changeable during the manufacturing process.
Composite materials can be infiltrated with molten metal by applying gas under pressure to thereby produce so called metal matrix composites (MMC) are formed. Prior to such an infiltration process, the metal must be heated above its melting point so as to be able to permeate into the preform of the composite material. At the temperatures generated herein, the oxygen in the surrounding air, however, reacts with the metal surface, forming oxides which are detrimental to the properties of the formed component. In addition, the preforms themselves can react with the oxygen in the air. As a result, substances, like oxides, oxynitrides, oxycarbides, or the like are formed in dependence on the composition of the preform. These substances which are formed mainly on the surface of the preform, may adversely affect the open porosity which is required for the infiltration process to work. Consequently, as a result of the growth of these oxide compounds, the diameter of the pores may decrease to such extent that the pressure created by gas is no longer sufficient to overcome the capillary forces acting on the liquid infiltration metal, so that the infiltration metal can no longer migrate into the preform. In the worst case scenario, the aforedescribed oxidation processes may even completely seal the open pores.
The reaction between preform material and air alters thus in addition the material properties of the preform, thereby effecting an unintentional alteration of the physical and thermal properties, on the one hand, and complicating or eliminating the reproducibility of the desired material properties of the composite.
In order to eliminate this drawback, there is the possibility to evacuate the preform before the infiltration metal is melted on, to thereby expel the oxygen from the pores.
In another known process, the air (and thus the oxygen) is expelled from the preform by an inert gas purge. This process, however, is rather unreliable and time-consuming since it takes a long time until the oxygen molecules are adequately purged from the pores of the preform or of the preform holder.
In summary, both these processes are complex and time-consuming.